The aims of this study were to evaluate genetic variations
in yield and reproductive developmental characters among peanut genotypes
in response to drought and relate these responses to pod yield under different
soil moisture. Eleven peanut genotypes were tested under three soil moisture
levels [field capacity (FC), 2/3 available soil water (AW) and 1/3AW]
in field experiments. Data were recorded for number of flowers, pegs (RSs),
immature pods and mature pods per plant, seed per pod, 100-seed weight
and pod yield at harvest. A drought tolerance index (DTI) for pod yield
was calculated as the ratio of pod yield under stress treatment to that
under well-watered conditions. The differences among water regimes were
significant for pod yield, number of RSs, immature pods and mature pods
per plant, seed per pod and 100 seed weight and differences among genotypes
were significant for all traits. Drought reduced pod yield, number of
RSs, pods and mature pods per plant. Early peak of flowering is important
for the formation of mature pods under drought conditions. Two different
strategies were used in maintaining high pod yield under drought. High
yield potential was important for ICGV 98348 and ICGV 98353, whereas low
pod yield reduction was important for ICGV 98305, ICGV 98303 and ICGV
98300. Tifton 8 showed the lowest pod yield and poor seed filling. High
RSs and well-filled mature pods were the most important traits contributing
to high pod yield in drought resistant genotypes.

Peanut (Arachis hypogaea L.) is an economically important food
legume. Most of the production areas are in arid and semi-arid tropic
regions, where peanut is grown mostly under rainfed conditions (Wright
and Nageswara Rao, 1994). Drought is the major abiotic stress factor affecting
yield and quality of rainfed peanut worldwide (Reddy et al., 2003;
Wright and Nageswara Rao, 1994). Therefore, the development of drought
resistant cultivars is of considerable importance. Peanut cultivars that
have a better ability to tolerate the effect of the reduced water supply
would help stabilize yields.

The relationships between pod yield and reproductive characters may be
altered by drought. So far, important characters determining pod yield
under drought and those affecting the response of peanut genotypes to
drought as far as reproductive characters and pod yield are concerned
are not well understood. Previous studies indicated that drought during
the vegetative phase has only small effect on growth and yield of peanut
(Awal and Ikeda, 2002; Nautiyal et al., 1999) due to the ability
to recover from early-season drought by initiating a flush of reproductive
growth after the relief of the stress (Nautiyal et al., 1999; Awal
and Ikeda, 2002). However, drought during the flowering and pod formation
phases is severely detrimental to the yield of peanut (Nautiyal et
al., 1999; Wright and Nageswara Rao, 1994) due to a decrease in flower
production if water stress occurred at flowering phase (Awal and Ikeda,
2002). Under water deficit conditions, pod yield might be affected as
a result of reduced pod growth and development (Reddy et al., 2003;
Chapman et al., 1993). Drought has also been show to decrease number
of mature pods and pod yield (Nautiyal et al., 1999).

According to Fernandez (1992), genotypes can be divided into four groups
based on their response to stress conditions: (i) genotypes producing
high yield under both water stress and non-stress conditions (group A),
(ii) genotypes with high yield under non-stress (group B) or (iii) stress
conditions (group C) and (iv) genotypes with poor performance under both
stress and non-stress conditions (group D). It is still not clear if selection
of drought tolerant peanuts should be carried out under conditions that
favor optimum yield, or under stress conditions, or under both conditions.
If selection is to be carried out under drought conditions, then more
information on the extent of stress is needed.

Pod yield can be considered as the sequential processes of flower production,
peg initiation, conversion of peg to pods and pod filling. To the best
of our knowledge, the contributions of reproductive characters such as
number of flowers, pegs and mature pods to pod yield stability under drought
have not been well researched.

Therefore, the objectives of this study were to evaluate genetic variations
in yield and reproductive developmental characters among peanut genotypes
in response to drought and relate these responses to pod yield under different
available soil water levels. This information should provide a better
understanding on how genotypes could achieve high yield under drought
and could important implications on breeding for drought resistance in
peanut.

MATERIALS AND METHODS

Experimental conditions and materials: The experiment was conducted
under field conditions at the Field Crop Research Station of Khon Kaen
University located in Khon Kaen province, Thailand (latitude 16° 28´
N, longitude 102° 48´ E, 200 m AMSL) during November 2003 to March
2004 and repeated during October 2004 to February 2005. Soil type is Yasothon
Series (loamy sand, Ocix Paleustults) with the following chemical attributes:
pH of 6.16-6.30, poor in organic matter (0.60-0.72%), total nitrogen (N)
(0.03-0.04%), available phosphorus (P) (35.32-45.84 ppm), potassium (K)
and calcium (Ca) (57.27 and 557.45 ppm, respectively).

Eleven peanut genotypes were used in this study (Table
1). Eight elite drought resistant lines obtained from ICRISAT (ICGV
98300, ICGV 98303, ICGV 98305, ICGV 98308, ICGV 98324, ICGV 98330, ICGV
98348 and ICGV 98353), one (Tifton-8) drought resistant line with a large
root systems (Coffelt et al., 1985) received from the United States
Department of Agriculture (USDA) and two (KK 60-3 and Tainan 9) released
cultivars commonly grown in Thailand. The lines from ICRISAT were identified as drought resistant because they produced high total biomass
and pod yield in screening tests under drought conditions (Nageswara Rao
et al., 1992; Nigam et al., 2003, 2005) KK 60-3 is a Virginia
type peanut in which pod yield is sensitive to drought and Tainan 9 a
Spanish-type peanut having low dry matter production (Vorasoot et al.,
2003, 2004).

A split plot design with four replications was used for both years. Three
soil moisture levels FC (10.55%), 2/3AW (8.48%) and 1/3AW (6.40%) were
assigned as main plots and 11 peanut genotypes were laid out in subplots.
Weather data for both years were obtained from a meteorological station
about 30 m away from the experimental site and are presented in Fig.
1.

Crop management: Land preparation was done by plowing the field
three times. Lime at 625 kg ha-1, phosphorus fertilizer as
triple superphosphate at 24.7 kg P ha-1 and potassium fertilizer
as potassium chloride at 31.1 kg K ha-1 were applied prior
to planting. Seeds were treated with captan (3a,4,7,7a-tetrahydro-2-[(trichloromethyl)thio]-1H-isoindole-1,3(2H)-dione)
at the rate of 5 g kg-1 seed before planting and seeds of the
two Virginia-type peanut cultivars (KK 60-3 and Tifton-8) were also treated
with ethrel (2-chloroethylphosphonic acid) 48% at the rate of 2 ml L-1
water to break dormancy. Three to four seeds were planted per hill and
the seedlings were thinned to two plants per hill at 14 days after sowing
(DAS). Rhizobium inoculation was done by applying a water-diluted commercial
peat-based inoculum of Bradyrhizobium (mixture of strains THA 201
and THA 205; Department of Agriculture, Ministry of Agriculture and Cooperatives,
Bangkok, Thailand) on the rows of peanut plants. Weeds were controlled
by an application of alachlor (2-cholro-2`,6`-diethyl-N-(methoxymethyl)
acetanilide 48%, w/v, emulsifiable concentrate) at the rate of 3 L ha-1
at planting and hand weeding during the remainder of the season. Gypsum
(CaSO4) at the rate of 312 kg ha-1 was applied at
45 DAS. Carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-ylmethylcarbamate
3% granular) was applied at the pod setting stage. Pests and diseases
were controlled by weekly applications of carbosulfan [2-3-dihydro-2,2-dimethylbenzofuran-7-yl
(dibutylaminothio) methylcarbamate 20% w/v, water soluble concentrate]
at 2.5 L ha-1, methomyl [S-methyl-N-((methylcarbamoyl)
oxy) thioacetimidate 40% soluble powder] at 1.0 kg ha-1 and
carboxin [5,6-dihydro-2-methyl-1,4-oxath-ine-3-carboxanilide 75% wettable
powder] at 1.68 kg ha-1.

Table 1:

Genotypes used and their branching patterns, growth
habit, maturity and botanical type

Subsurface drip-irrigation system (Super Typhoon®; Netafim
Irrigation equipment and Drip systems, Tel Aviv, Israel), a distance of
20 cm between emitters was installed with a spacing of 50 cm between driplines
at 10 cm below the soil surface mid-way between peanut rows to supply
water to the crop and fitted with a pressure valve and water meter ensured
the uniform supply of measured amount of water across each plot. Soil
moisture was initially maintained at field capacity (93.1 mm in 60 cm
depth) until 21 DAS in all treatments to support crop establishment. After
21 DAS, stress treatments 2/3 AW and 1/3 AW were imposed by withholding
irrigation until the soil moisture levels at 0-60 cm of soil depth reduced
to predetermined levels of 75 and 56 mm in 60 cm depth in 2/3 AW and 1/3
AW treatments, respectively. Soil moistures in stress treatments reached
2/3 and 1/3 AW at 28 DAS and 35 DAS, respectively. The 2/3 AW and 1/3
AW water regimes were maintained not lower than 1% of the predetermined
levels until harvest by controlling water input through the sub-soil drip
system. In maintaining the specified soil moisture levels, water was added
to the respective plots by subsurface drip-irrigation based on crop water
requirement and surface evaporation which were calculated following the
methods described by Songsri et al. (2008).

Calculation of total crop water use for each water treatment was calculated
as the sum of transpiration and soil evaporation. Transpiration (T) was
calculated using the formula:

ETcrop = ETo
x Kc

(1)

Where:

ETcrop

=

Crop water requirement (mm day-1)

ETo

=

Evapotranspiration of a reference plant under specified conditions
calculated by pan evaporation method

Kc

=

The crop water requirement coefficient for peanut, which varies
with genotype and growth stage

Surface evaporation (Es) was calculated as:

Es = β
x (Eo/t)

(2)

Where:

Es

=

Soil evaporation (mm)

β

=

Light transmission coefficient measured depending on crop cover

Eo

=

Evaporation from class A pan (mm day-1)

t

=

Days from the last irrigation or rain (day)

Data collectionsWeather parameters: There was significant rainfall (71 mm) during
73-75 DAS in the dry season 2003-04, while the crop in the dry season
2004-05 had no interference from rain (Fig. 1). The
seasonal mean maximum and minimum air temperature ranged between 31.0
and 18.0°C in 2003-04 and 32.0 and 19.0°C in 2004-05, being lower
during 1-45 DAS in 2003-04. Daily pan evaporation ranged from 0.8 to 9.9
mm in 2003-04 and 2.2 to 8.3 mm in 2004-05. The seasonal mean solar radiation
17.6 in 2003-04 and 17.7 MJ m-2 day-1 in 2004-05,
were observed.

Soil moisture and plant water status: Soil moisture was measured
by gravimetric method at planting and harvesting at the depths of 0-5,
25-30 and 55-60 cm. The measurement at planting was for calculating the
correct amount of water to be applied to the crop and the measurement
at harvest was for calculating the water use of the crop. The soil water
status was also monitored at 7 day intervals using a neutron moisture
meter (Type I.H. II SER. N° N0152, Ambe Didcot Instruments Co. Ltd.,
Abingdon, Oxon, UK). An aluminum access tube was installed between rows
in each plot. Sixteen-second neutron moisture meter readings were made
at least weekly from a depth of 0.3 to 0.9 m at 0.3 m intervals.

Leaf water potential (LWP) and leaf relative water content (RWC) were
used to evaluate plant water status and were measured at 37, 67 and 97
DAS. A pressure chamber (PMS instrument co., USA) was used to determine
LWP using the second fully expanded leaf from the top of the main stem
and one leaf for each plot at 1000-1200 h. At the same time as the leaf
relative water content (RWC) was recorded using five leaves of the second
fully expanded leaf from the top of the main stem for each plot and the
formula suggested by Turner (1986) as follows:

RWC (%) = [(FW-DW)/(TW-DW)]
x 100

(3)

Where:

FW

=

Sample fresh weight

TW

=

Sample turgid weight

DW

=

Sample dry weight

Number of reproductive parts: The number of flowers was recorded
daily on five tagged plants from each plot during the morning (600-800
h, Thailand standard time) from the date of first flowering until harvest.
The numbers of reproductive parts were recorded at harvest as the number
of pegs (reproductive sink number; RSs = hanging pegs+pods), number of
total pods (immature and mature pods) and number of mature pods per plant
(mature pods was separated from immature pods, which were identified by
their shriveled seeds and dark internal pericarp color). Number of seeds
per pod was recorded on the plants that had been tagged for flower counts
at final harvest.

Pod yield and seed size: At maturity, plots of four rows with
4.0 m in length (8 m2) were dug by hand, roots were cut at
crown level and discarded and total above-ground biological yield and
marketable pod yield were determined. Mature pods were weighted after
air drying to approximately 8% moisture content. Other plant parts except
for mature pods were oven-dried at 80°C for 48 h and their dry weights
were recorded. Seed size (100 seed weight) was determined from harvested
seeds.

Drought Tolerance Index (DTI), as suggested by Nautiyal et al.
(2002), was calculated for pod yield (DTI (PY)) as the ratio of pod yield
under stressed treatments (2/3 AW or 1/3 AW) to that under well-watered
(FC) condition as follow:

Statistical analysis: Individual analysis of variance was performed
for each year followed a split plot design (Gomez and Gomez, 1984). Homogeneity
of variance was tested for all characters and combined analysis of variance
of two-year data was performed. Calculation procedures were done using
MSTAT-C package. Due to the significance of year x genotype as well as
water regime x genotype interaction (Table 2), data
for each year and each water regime were analyzed separately according
to a randomized complete block design (RCBD) and Duncan`s multiple range
test was used to compare means (Gomez and Gomez, 1984). Correlation coefficients
between pod yield and DTI for pod yield and reproductive traits were calculated
to assess their relationship.

Multiple-linear regression was used to determine the relative contribution
of reproductive traits to pod yield under FC, 2/3 AW and 1/3 AW. The analysis
was based on the following statistical model (Gomez and Gomez, 1984):

Table 2:

Mean squares from the combined ANOVA for pod yield,
number of flower, pegs, pods and mature pods per plant, seed per pod
and 100 seed weight under three water regimes of 11 genotypes in dry
seasons 2003-04 and 2004-05

SOV = Source of variation, DF = Degree of freedom, *p<0.05,
**p<0.01

Yi = α
+ β1X1i + β2X2i
+ β3X3i + β4X4i
+ δi

(4)

Where:

Yi

=

Pod yield of genotype i

α

=

The Y intercept

X1i, X2i, X3i and X4i are
number of flower, RSs, pods and mature pods of genotype i, respectively,
β1, β2, β3 and β4
are regression coefficients for the independent variables X1,
X2,X3 andX4 and
δi is the associated deviation from regression.

The analysis was carried out by fitting the full model first and then
determining the relative importance of the individual independent variables.
A sequential fit was then performed by fitting the more important variable
first. The relative contributions of the individual independent variables
to pod yield under FC, 2/3 AW and 1/3 AW were determined from the percentages
of regression sum of squares due to the respective independent variables
to total sum of squares in the sequential fitted analysis.

RESULTS AND DISCUSSION

Monitoring of soil moisture and water status in plant: Soil water
status showed reasonable management of soil moistures (Fig.
2). A clear distinction among soil moisture levels was noted at 30
cm of soil depth except in the year 2003-04, when rainfall of 71 mm at
73-75 DAS, which caused high soil moistures in the drought treatments.
Soil moistures at 90 cm depth were similar among treatments because the
amount of water to apply in each treatment was calculated for 0-60 cm.

Relative water content (RWC) and leaf water potential (LWP) were significantly
lower in the plants experiencing soil-moisture-deficit stress than their
respective controls (Fig. 3). The highest LWP and RWC
were observed for soil moisture at FC followed by 2/3 AW and 1/3 AW, respectively.
Observations found visual wilting in 2/3 AW and more severe wilting in
1/3 AW in the afternoon. RWC and LWP of the plants in the 1/3 AW treatment
were extremely low at 97 DAS.

Effect of available soil water on pod yield: Drought significantly
reduced pod yield (Table 3). The peanut genotypes were
grouped depending on their similar and consistent patterns for pod yield
in responses to drought. ICGV 98353 and ICGV 98348 were grouped together
because they had consistently high pod yield under both FC and drought
conditions (group A) (Table 3). The yield stabilizing
strategies for these genotypes should be largely from their high starting
yield at FC and in minor part from their relatively low reductions (high
DTI). No genotype was observed to have high pod yield under non-stress
conditions only (group B). ICGV 98305, ICGV 98303 and ICGV 98300 exhibited
high pod yield under drought only (group C). The reductions in pod yield
of these genotypes were somewhat lower than those of group A and low reduction
(high DTI) in pod yield was more important for stabilizing the yield of
these genotypes under drought. The genotypes with poor performance for
pod yield under both non-stressed and stressed conditions were Tifton-8
and Tainan 9 (group D). These genotypes had the lowest potential yield.
The reduction in pod yield of these genotypes was also relatively high,
indicating that they were most sensitive to drought.

The results indicated that two possible strategies of drought resistance
may be useful for explaining drought resistance in these peanut genotypes.
Genotypes with high pod yield under drought conditions should be of either
(i) high pod yield under well-watered conditions (e.g., ICGV 98348 and
ICGV 98353) or (ii) ability to maintain a low rate of yield reduction
under increasing stressed (high DTI) (e.g., ICGV 98305, ICGV 98303 and
ICGV 98300). However, the strategies for maintaining high yield under
mild and severe drought were quite different. Under 2/3 AW both high potential
and low reduction were equally important, but in 1/3 AW low reduction
strategy was more important than yield potential (Fig. 4).

Pod
yield (t ha-1) and drought tolerance index (DTI) for pod
yield of 11 peanut genotypes grown under different water regimes at
harvest in dry seasons 2003-04 and 2004-05

Mean in the same column with the same letter(s) are not significantly
different by DMR at p≤0.05, DTI for a genotype were calculated
by the ratio of stressed (2/3 AW or 1/3 AW)/non stressed (FC) conditions

Fig. 3:

Leaf
water potential (LWP) (a, b) and relative water content (RWC) (c,
d) in three available soil water regimes (FC, •; 2/3 AW,
and 1/3 AW, Δ) at 37, 67 and 97 DAS during the 2003-04 and 2004-05,
seasons

Flowering
patterns of 11 peanut genotypes grown under three available soil water
regimes in 2003-04 and 2004-05

Effect of available soil water on flowering behaviors and reproductive
parts: The flowering behaviors in 2003-04 and 2004-05 were different
in both pattern and flower number. The first flowers for all genotypes
in 2004-05 appeared between 21-28 DAS (Fig. 5) which
was seven days earlier than observed in 2003-04. The delay of the first
flowering in 2003-04 was caused by lower temperature during December and
the higher number of flowers in 2003-04 was caused by higher soil moisture
for a short duration because of rainfall.

The patterns of flower production for specific genotypes under different
water regimes in the same year were similar, but they were different between
years. Flowering patterns in 2003-04 were bi-modal, whereas flowering
patterns in 2004-05 were uni-modal. The difference in the flowering patterns
between years was due to rainfall during pod filling (73-75 DAS) in the
year 2003-04 resulting in a second peak of flowering in all genotypes.
Nautiyal et al. (1999) also found flush flowering of Spanish type
peanuts after a relief from drought. The flowering patterns were classified
into two types, (i) long duration flowering (KK 60-3 and Tifton-8) and
(ii) short early peak flowering (Fig. 5). Our findings
supported the typical flowering patterns of Spanish and Virginia peanuts
reported by Coolbear (1994). However, the peaks of Spanish genotypes in
this study were more skewed toward the left (earlier peak flowering).

When the representatives of different type peanuts were compared, Tainan
9 (Spanish) and Tifton-8 (Virginia) were similar for low flower number
per plant, but they differed in flowering patterns. Tainan 9 had the shortest
and earliest peak flowering, whereas Tifton-8 had lower and longer duration
flowering than did Tainan 9 and other genotypes. Tainan 9 also had higher
pod yield than Tifton-8 under water stress. The difference in flowering
patterns might be more important than total flower number in determining
pod yield under drought. Wright and Nageswara Rao (1994) also reported
that reduction in flower number arising from water deficits do not directly
influence pod yield since only 15-20% of flowers result in pods that contribute
to yield.

Wright and Nageswara Rao (1994) reported that some genotypes with high
flower production at FC did not produce high number of pegs and pods under
stress, but some genotypes with lower flower production at FC could produce
similar numbers of pegs and pods under drought. Pegging and pod set responses
of various peanut genotypes under drought varied substantially, leading
to large differences in pod yield and the reductions in pod yield also
varied among peanut genotypes (Nautiyal et al., 1999; Nageswara
Rao et al., 1989). However, the genotypes producing the lowest
number of flowers under normal conditions rarely produced high pod yield
under water stress.

Peanut genotypes differed substantially in their flowering, pegging and
pod set in response to drought (Table 4). For instance,
ICGV 98348, ICGV 98300 and ICGV 98330 had high flower production, number
of pegs per plant and number of pods per plant under well-watered conditions
in both years. ICGV 98324 and ICGV 98353 had moderate flowering production,
but produced high number of pegs and pods at both stressed and well-watered
levels. Under water stressed conditions, ICGV 98348 and ICGV 98353 maintained
high RSs in both years and ICGV 98324 showed high RSs only in 2004-05
at 2/3AW and 1/3AW (Table 4). Tainan 9, KK 60-3 and
Tifton-8 were the genotypes producing the lowest number of flowers under
normal conditions and they could not produce high pod yield under drought.
ICGV 98324, ICGV 98348 and ICGV 98353 were characterized by maintaining
higher number of flowers and pegs under drought.

The difference in flowering behavior is also determined by their botanical
groups, Spanish and Virginia. Spanish botanical type had higher total
numbers and earlier peak flowering than did Virginia type (KK60-3 and
Tifton-8). With regard to the production of reproductive parts contributing
to yield under stressed conditions, Spanish type peanuts were in general
superior to Virginia type peanuts. Spanish type peanuts were also superior
to Virginia type peanuts for flowering pattern with a flush of flowers
that developed to mature pods. Most flowers at latter stages did not fertilize
or became hanging pegs and immature pods. In general, flowers and pegs
were abundant under non-stress and stressed conditions and they did not
seem to be the important characters limiting yield under drought conditions.

Effect of available soil water on yield components: Water stress
reduced the number of mature pods, seeds per pod and seed size in both
years (Table 5). In general, the reductions were high
for mature pods and 100-seed weight (low DTI), but lower for number of
seeds per pods. Higher reductions in yield and yield components were found
in 2004-05 than in 2003-04. The differences between years is likely due
to rainfall in 2003-04 that enhanced performance of genotypes in stressed
treatments especially at 1/3 AW.

Peanut genotypes may use different strategies to develop high number
of mature pods and pod yield. We related number of mature pods to flowering
and pegging to understand how they reflected number of mature pods under
stressed conditions. However, the division of peanut genotypes was not
clear because of less divergence of the genotypes for number of mature
pods and genotype x water regime interactions. As the genotypes were not
significantly different for number of mature pods at FC in 2004-05, special
attention was given to the performance under stressed conditions especially
in 2004-05.

The genotypes with high number of mature pods under both non-stressed
and stressed conditions (group A) were ICGV 98303, ICGV 98353 and ICGV
98348. These genotypes had high number of mature pods under stressed condition
because of high numbers of total pods and pegs and number of mature pod
at FC. ICGV 98348 also had the highest number of flowers, whereas the
others
had somewhat lower number of flowers. High number of flowers is advantageous
to produce higher mature pods, but it is not necessary for group A.

Table 4:

Number
of flowers, pegs and pods plant-1 of 11 peanut genotypes
grown under different water regimes at harvest in dry seasons 2003-04
and 2004-05

Mean
in the same column with the same letter(s) are not significantly different
by DMR at p≤0.05

No genotype performed well for number of mature pods under non-stressed
conditions only (group B) and none had high number of mature pods under
stressed conditions only (group C). The genotypes with low number of mature
pods under both non-stressed and stressed conditions (group D) were Tifton-8
and KK 60-3 (Table 5). These genotypes had low number
of mature pods under stressed conditions due to low numbers of flowers
and pegs.

There were no differences among peanut genotypes for number of seeds
per pod at any water level in 2003-04, but there were significant differences
at 2/3 AW and 1/3 AW in 2004-05. In 2004-05, ICGV 98353 showed high numbers
of seeds per pod and Tifton-8 had the lowest number of seeds per pod under
water stressed conditions (Table 5). Although the genetic
variation was low, clearer differences were observed under drought conditions,
indicating differences in drought sensitivity among these peanut genotypes
for number of seeds per pods. Drought reduces seed filling and only the
first seeds in some pods are well-filled (limited full pod load), forming
taper-shaped pods in the later-developing pods (Wright and Nageswara Rao,
1994).

Using the criterion of grouping mentioned above (Fernandez, 1992), the
genotypes showing the most consistent patterns for seed size in both years
were grouped together. KK 60-3 was classified as group A, Tifton-8 as
group B, ICGV 98324 as group C and ICGV 98348 as group D (Table
5). KK 60-3 and Tifton-8 had high seed size under non-stressed conditions.
These Virginia type large-seeded peanuts also had higher rates of reduction
in seed size than did the medium-seeded genotypes under water stressed
conditions. The rate of reduction in seed size of Tifton-8 was higher
than that of KK 60-3, making it perform poorer under water stressed conditions.
ICGV 98324 was not among the largest under non-stressed conditions, but
under stressed conditions it had seed size larger than others.

Table 5:

Number
of mature pods plant-1, seed pod-1 and 100 seed
weight of 11 peanut genotypes grown under different water regimes
at harvest in dry seasons 2003-04 and 2004-05

Mean
in the same column with the same letter(s) are not significantly different
by DMR at p≤0.05

Larger seeded genotypes are more sensitive to environmental changes than
smaller seeded genotypes (Vorasoot et al., 2003). Large seeded
peanuts typically have more severe reduction in seed size and yield under
stressed conditions. Seed size in 2003-04 was larger than in 2004-05.
Difference between years was caused by rainfall in 2003-04 that promoted
seed filling of the water-starved plants. Peanut is unlike other grain
legumes in that its yield depends on photo-assimilates during pod growth
rather than re-translocation (Chapman et al., 1993).

Chapman et al. (1993) Observed that the number of mature pods
is important for yield under most, if not all, growing conditions, while
seed size is more important under stressed conditions than normal conditions.
Although there are compensations among yield components, these characters
might be synergistic to each other to produce higher yield. The results
might also imply that the genotypes maintaining either high number of
mature pods or large seed size or combination of both would be advantageous
under drought conditions.

Relationship between reproductive characters and pod yield: Most
correlation coefficients between pod yield and flower number per plant
were not significant except under 1/3 AW conditions (r = 0.51; p≤0.05)
and the correlation between numbers of pegs and pods per plant were significant
at well-watered and 2/3 AW conditions only
(Table 6). Significant correlation coefficient were
found for number of mature pods per plant and pod yield at mild (0.63;
p≤0.01) and severe (0.87; p≤0.01) drought and correlation coefficients
between mature pods number and DTI were also significant at 2/3 AW (0.44;
p≤0.05) and highly significant at 1/3 AW (0.76; p≤0.01). Significant
correlation coefficients were observed between DTI (PY) and seed per pod
(0.45; p≤0.05) and between DTI (PY) and seed size (0.57; p≤0.01)
at 1/3 AW conditions. Number of mature pods per plant seemed to play an
important role in maintaining high pod yield under drought especially
under severe stressed conditions (Table 7). If selection
based on high yield under drought conditions is to be practiced, number
of mature pods should be considered as surrogate trait for pod yield because
this trait is more simple and has lower genotype by environment interaction
than pod yield (Table 2). Visual screening for high
number of pods can be practiced in the fields in early generations of
segregating populations. However, pod yield is still necessary in more
advanced generations when replicated trails are normally practiced.

Table 6:

Correlation
coefficients among the pod yield and drought tolerance index for pod
yield (DTI (PY)) and number of flowers, pegs, pods, mature pods per
plant, seed per pod and 100 seed weight of 11 peanut genotypes under
three water regimes grown in the field during 2003-04 and 2004-05

*
and **significant at p≤0.05 and significant at p≤0.01, respectively

Table 7:

Contributions
of reproductive characteristics to pod yield under FC, 2/3AW and 1/3
AW conditions of dry seasons 2003-04 and 2004-05

*
and **significant at p≤0.05 and significant at p≤0.01, respectively

In summary, High numbers of flowers and pegs are advantageous but not
necessary for high yield under drought conditions. Early peak of flowering
is important for the formation of mature pods and number of mature pods
is the most important character determining pod yield under both non-stressed
and stressed conditions. Seed size is also important for pod yield under
drought but in lesser extent. High pod yield under non-stressed conditions
is most important for high pod yield under drought conditions in ICGV
98348 and ICGV 98353. This is because of high fruit set and well-filled
mature pods. Tifton-8 had the lowest pod yield under water stressed conditions
because of both its low pod yield under normal conditions and high reduction
in pod yield. Study findings showed that high pod yield under non-stressed
conditions are important for high pod yield under drought conditions in
some genotypes, whereas low reduction in pod yield under drought conditions
is more important in others. This information should help breeders to
better understand the factors that lead to higher pod yield under drought
and may help breeders to formulate more effective and efficient breeding
strategies for improving drought resistance in peanut.

ACKNOWLEDGMENTS

The authors are grateful for the financial support of the Royal Golden
Jubilee Ph.D Program (Grant No. PHD/0190/2544) and the Senior Research
Scholar Project of Professor Dr. Aran Patanothai under the Thailand Research
Fund and also support in part by the Peanut Improvement Project of Khon
Kaen University. Very thankful acknowledgments have been given to Dr.
S.N. Nigam (ICRISAT, India) for the donation of peanut seed. We gratefully
acknowledge Dr. Viboon Pensuk (Ratchapat Udonthanee University) to provide
a pressure chamber for LWP measurement. We thank the work of many people
in field data collection and processing.

Fernandez, G.C.J., 1992. Effective selection criteria for assessing plant stress tolerance. Proceedings of the International Symposium on Adaptation of Vegetables and other Food Crops in Temperature and Water Stress, August 13-16, 1992, Shanhua, Taiwan, pp: 257-270.